The present disclosure relates to a photolithography apparatus comprising a projection system and a method for controlling an image size of a projected substrate pattern.
Photolithography is a process used in micro- or nanofabrication to pattern parts a substrate. Typically, it uses light to transfer a mask pattern from a mask to a light-sensitive chemical (“photoresist”) on the substrate. For example, a photolithography apparatus comprises or couples to an illumination system with a light source and/or beam shaper for providing the light to the mask. The mask pattern is imaged by a projection system onto the substrate. The projection system comprises optical elements such as curved mirrors and/or lenses to control the magnification, position and/or focusing of the projected image.
As used in the microlithography industry, such optical elements typically achieve high resolution to resolve structures in the mask, e.g. close to the theoretical diffraction limit. When applying successive layers to a substrate, the images typically needs to be superimposed to an accuracy that is a fraction of that resolution. However, when an environmental condition such as the temperature or air pressure changes, the scale and/or focusing of the image may be affected. This can be further complicated when layers are applied by different machines. Accordingly, it is desired to accurately control the relative image size of a projected substrate pattern.
In a typical projection system, control of the image size can be achieved by moving a lens along the optical axis for adjusting the magnification. However, this may generally also influence a position of the image plane thus leading to a blurring of the projected substrate pattern. The shift of the image plane can be compensated by simultaneously moving a second adjustment lens or even the position of the substrate itself. However, this requires two adjustments that are generally unequal and difficult to coordinate, e.g. non-linear.
U.S. Pat. No. 6,816,236 discloses a projection optical system that includes six lens groups, four of which are positioned in pairs symmetrically about a stop. The second and fifth lens groups, in order from the object side, may be positioned symmetrically about the stop but are also adjustable asymmetrically about the stop in order to adjust the magnification of the projection optical system. The first and sixth lens groups, in order from the object side, function to make the projection optical system substantially telecentric on both the object side and the image side, respectively. A projection and light exposure apparatus automatically detects the image magnification and, based on the detection result, adjusts the positions of the second and fifth lens groups as a unit so as to maintain a specified magnification.
Unfortunately, the prior art solution is limited to lens groups that are positioned in pairs symmetrically about a stop. Furthermore, because the prior art relies on controlling the magnification by means multiple lens groups, the imaging may be affected by the relative positioning of said groups. For example the second lens group may shift with respect to the fifth lens group and/or with respect to the third or fourth lens groups therein between as a result of temperature fluctuations in the system. Also when the second and fifth lens groups are not manufactured identical, the image may deteriorate.
Accordingly, there is a desire for a photolithography apparatus comprising a more versatile and robust projection system for controlling the relative image size of a projected substrate pattern.